A digital buffer (or a voltage buffer) is an electronic circuit element that is used to isolate the input from the output, providing either no voltage or a voltage that is same as the input voltage. It draws very little current and will not disturb the original circuit. It is also called as unity gain buffer or a because it provides a gain of 1, which means it provides at most the same voltage as the input voltage, serving no amplification function. Usage of buffer in Physical Design 1. Setup Violation fixes 2. Hold Violation fixes 3. Max Trans Violation Fixes 4. xtalk reduction
Inrush current, input surge current or switch-on surge is the maximum, instantaneous input current drawn by an electrical device when first turned on. Alternating current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform. Power converters also often have inrush currents much higher than their steady state currents, due to the charging current of the input capacitance. The selection of overcurrent protection devices such as fuses and circuit breakers is made more complicated when high inrush currents must be tolerated. The overcurrent protection must react quickly to overload or short circuit faults but must not interrupt the circuit when the (usually harmless) inrush current flows.
There are several ways to fix the transition time violations. 1) Increase the driver size. 2) Break the nets in the case of long nets. 3) Break the large fanout by duplicating drivers or with buffering. 4) Change the VT if option available(changing drivers from hvt to svt or lvt). 5) Reduce the load by downsizing the cells(special cases) after the looking the timing impact on the design. 6) Change the Load to hvt because hvt has higher lib limit.
•What are well tap cells, what are end cap cells and its usage
•What are the inputs to CTS
•What sanity checks are to be done in SDC file
•What is de-rating
•What is clock skewing and what is a useful skew
•how many clocks are used in your project
•what is the maximum frequency
•what is latency
•what is clock generation point, clock distribution point, clock end point
•why is the setup check in next cycle and hold checked at same time
•what is the width and spacing rules for clocks
•what are the NDR for clocks
•what are NDR and DRC checks done at each stage of project
•what is OCV, antenna violation and what are measures taken
•what is antenna ratio
•what is RC extraction and when do you do this
•if you have congestion after CTS and you are not allowed to change the placement how do you proceed
•does set up fixes cause hold issues and vice versa
•what is slack
•what happens if there is any floating pin and it is left without any care
•what is electro migration
•what is signal integrity and cross talk
•what are the preventions taken at each stage to resolve cross talk issue
•what is shielding
•what is Physical verification, formal verification
•what are the DRC checks made in project
•what is LVS what information is obtained from LVS
•what are the signoff checks and which tools are used for it
•what are the inputs to STAR RC and prime time
•what is cloning
•what happens if any of the input files are missed to IC compiler
•what is RTL , gate level differences
•What does synthesis team do?
•What are libraries needed for tool, what is difference between logic and physical library
•What is a UPF file, is it used in your project
•What ae filler cells
•What are ECOs , how many Eco are implemented
•What ae the DRC fixed
•What is low power design
•What is clock and power gating
•Tell me about yourself in brief •Inputs to PNR •Do you have knowledge on synthesis •What are the validations and sanity checks you do on the outputs received from synthesis team •Which file will you need to check if you see black box in the screen •Which file you need to check if you see any floating pins and whom should you report in such case •What does .tf file, .db file, .sdc file, .spef file .v files include •What is floor plan and what is done as part of floor plan •How do you fix placement of RAMs •What is utilization factor and Area •What is fly line analysis •How do you decide the spacing between the macros and standard cells •What are tie cells and can the size of array of tie cells be either increased or decreased? –No •Which layers are preferred for power routing and why? •Which is preferred to be the outer most layer or top layer •What is the UF in floor plan •What is the skew achieved in your project and what is the allowed skew •What is local skew and global skew difference •What are the tools used for PNR •What is high fan out synthesis •What is placement and what do we do in this step •What is NLDM •What is congestion and timing closure •What special physical cells are used in your project •Purpose of power domains, level shifter cells, isolation cells, Always on cell •Where do we use Always On buffers •How is drive strength and delay relation •What is inversion temperature •How is delay and temperature relation •How is threshold voltage and temperature variations related •What happens when you do congestion driven placement •Where are the buffers placed? What is the functionality of buffers •How do buffers speed up the signal in data or in clock even if it adds delay to the path •What is set up and hold violation •What are the ways to fix the set up violation •Ways to fix hold violation •What is STA and which tool is being used for STA • What are the optimizations done in placement stage •What are the power domains in your project •What is switchable power domain
In this position, the individual will be responsible for driving Place-And-Route flow for Low Power Designs from netlist to GDS. Responsibilities will include complete ownership of sub chip PnR environment, clock tree design and driving Place-And-Route flow for full chip convergence and timing closer.
This position requires a Master’s Degree in Electrical Engineering or Computer Science with a minimum of 4 to 7 years of hands on experience in sub chip Place-And-Route flow with emphasis on 28nm and low power designs. Proficiency in SoC Encounter / Innovus / ICC / ICC II and experience in PERL/TCL/Shell scripting is a must.
The individual must have hands on experience with multiple low power hierarchical and flat ASICs at 28 nm and below nodes. Having exposure to Physical Verification flow for DRC and LVS closure for blocks is preferred. Ability to work with minimal supervision and drive to exceed expectations is a must. Good verbal and written communication skills are required.
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